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Atoms of the same element always have the same number of protons (i.e. same atomic number). But, atoms of the same element can have different numbers of neutrons. We call these isotopes. Hydrogen has three isotopes:

Illustrative background for ProtiumIllustrative background for Protium ?? "content


  • Protium is a hydrogen atom with 1 proton and 0 neutrons.
  • 99.98% of hydrogen atoms are protium.
  • It is used in hydrogen fuel cells and the production of plastics.
Illustrative background for DeuteriumIllustrative background for Deuterium ?? "content


  • Deuterium is a hydrogen atom with 1 proton and 1 neutron.
  • Around 0.02% of hydrogen atoms are deuterium.
  • It is used in nuclear fusion.
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  • Tritium is a hydrogen atom with 1 proton and 2 neutrons.
  • It is very rare.
  • It is used in thermonuclear fusion weapons.


A nuclide is a type of isotope.

Illustrative background for Definition of a nuclideIllustrative background for Definition of a nuclide ?? "content

Definition of a nuclide

  • A nuclide refers to a specific nucleus that contains a certain number of protons and neutrons.
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  • A carbon nuclide with 6 protons and 6 neutrons is different to a carbon nuclide with 6 protons and 7 neutrons. But, both are isotopes of carbon.

Carbon Dating

We can use our knowledge of isotopes to calculate the age of fossils.

Illustrative background for Radioactive carbon-14Illustrative background for Radioactive carbon-14 ?? "content

Radioactive carbon-14

  • All living organisms have the same amount of carbon-14 atoms as a percentage of all carbon isotopes.
  • Once the organism dies, it no longer absorbs carbon from the atmosphere.
  • Carbon-14 is radioactive and so will decay over a known half life.
    • So, the older a fossil is, the fewer carbon-14 isotopes it will contain and the less radiation it emits.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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